Eyewitness accounts from the Syrian massacre have emerged, describing gunmen slaughtering whole families in their homes and targeting the most vulnerable in poor farming villages.

UN special envoy Kofi Annan met with President Bashar Assad in Damascus to try to salvage what was left of a peace plan, which since being brokered six weeks ago has failed to stop any of the violence on the ground.

Survivors of the Houla massacre blamed pro-regime gunmen for at least some of the carnage as the killings reverberated inside Syria and beyond, further isolating Assad and embarrassing his few remaining allies.

"It's very hard for me to describe what I saw, the images were incredibly disturbing," a Houla resident who hid in his home during the massacre said. "Women, children without heads, their brains or stomachs spilling out."

He said the pro-regime gunmen, known as shabiha, targeted the most vulnerable in the farming villages that make up Houla, a poor area in Homs province. "They went after the women, children and elderly," he said, asking that his name not be used out of fear of reprisals.

Assad's government often deploys fearsome militias that provide muscle for the regime and carry out military-style attacks. They frequently work closely with soldiers and security forces, but the regime never acknowledges their existence, allowing it to deny responsibility for their actions.

UN peacekeeping chief Herve Ladsous said there are strong suspicions that pro-Assad fighters were responsible for some of the killings, adding that he has seen no reason to believe that "third elements" - or outside forces - were involved, although he did not rule it out.

The Syrian regime has denied any role in the massacre, blaming the killings on "armed terrorists" who attacked army positions in the area and slaughtered innocent civilians. It has provided no evidence to support its narrative, nor has it given a death toll.

Following his meeting with Assad, Annan called on the government and "all government-backed militias" to stop military operations and show maximum restraint. He also called on the armed opposition to stop all violence.

"We are at a tipping point," Annan told reporters in Damascus. "The Syrian people do not want the future to be one of bloodshed and division."